Responding to the enviroment

Plants respond to stimuli

tropism is a directional growth response in which the direction of the response is determined by the direction of the external stimulus.

Tropisms include:

  • Phototrophism - shoots grow towards light (they are positively phototrophic), which enables them to photosynthesise.
  • Geotrophism - roots grow towards the pull of gravity. This anchors them in the soil and helps them to take up water, which is needed for support, as a raw material for photosynthesis and to help cool the plant. There will also be minerals such as nitrates, in the water, needed for the synthesis of amino acids.
  • Chemotrophism - on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
  • Thigmotrophism - shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
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What controls plant responses?

Hormones coordinate plant responses to enviromental stimuli. They are oftern referred to as plant growth regulators.

When hormones reach their target cells they bind to receptors on the plasma membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular celss. This makes sure that the hormones only act upon the correct tissue.

Hormones can move around the plant in any of the following ways:

  • Active transport.
  • Diffusion.
  • Mass flow in the phloem sap or xylem vessels.

Some hormones can amplify each others effects (synergy), and some can cancel out each other's effects (antagonism). Hormones can influence cell division, cell elongation, or cell differentiation.

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Plant growth

Because of the cell wall growth can only happen in particular place in the plant, where there are groups of immature cells that are still capable of dividing. These places are called meristems.

  • Apical meristems are located at the tips or apices of roots and shoots, and are responsible for the roots and shoots getting longer.
  • Lateral bud meristem are found in the buds. These could give rise to side shoots.
  • Lateral meristem are found in a cylinder near the outside of roots and shoots and re responsible for the roots and shoots getting wider.
  • In some plants, intercalary meristems are located between the nodes. Growth between the nodes is responsible for the shoot getting longer.

Auxins stimulate shoot growth by causing cell elongation. They are produced at the apex and travel either by diffusion or active transport to the cells in the zone of elongation. The extent to which cells elongate is proportional to the concentration of auxins. Auxin increases the strectchiness of the cell wall by promoting the active transport of hydrogen ions, by an ATPase enzyme on the plasma membrane, into the cell wall. The resulting low pH provides optimum conditions for expansins to work. These enzymes break bonds within the cellulose so that the walls become less rigid and can expand as the cell takes in water.

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What causes phototropisms?

In a phototropic response, a shoot bends towards a light source, this is because the shaded side elongates faster than the illuminated side, which pushes the end of the shoot towards the light.

The light shining on one side of the shoot causes the auxins to be transported to the shaded side, where they promote an increase in the rate of elongation, making the shoor bend towards the light.

How the light causes redistribution of auxin is still uncertain. Two enzymes have been identified (phototropin 1 and phototrophin 2). Their activity is promoted by blue light (wavelength 400-450nm). Hence there is a lot of phototrophin 1 activity on the light side, but progressively ledd activity towards the dark side. This gradient is thought to cause the redistribution of auxins.

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Shedding leaves

Cytokinins stop the leaves of deciduous trees senescing by making sure the leaf acts as a sink for phloem transport, guaranteeing a good supply of nutrients. However, if cytokinin production drops, the supply of nutrients dwindles and senescence begins, followed by the leaves being shed (abscission).

Usually auxin inhibits abscission by acting on cells in the abscission zone. However:

  • Leaf senecence causes auxin production at the tip of the leaf to drop.
  • This makes cells in abscission zone more sensitive to another growth substance called ethene.
  • A drop in auxin concentration also causes an increase in ethene production.
  • This in turn increases production of the enzyme cellulase, which digests the walls of the cells in the abscission zone, eventiually separating the petiole from the stem.
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Apical dominance

Apical dominance is when the growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot.

Abscisic acid inhibits bud growth. High concentrations of auxins in the shoot may keep abscisic acid levels high in the bud. When the tip is remove (the source of auxin) the abscisic acid concentrations drop and the bud starts to grow.

Cytokinins promote bud growth - directly applying cytokinins to buds can override the apical dominance effect. High concentrations of auxins make the shoot apex a sink for cytokinins produced in the roots. When the apex is removed cytokinins spread more evenly around the  plant, promoting growth in the buds.

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Gibberellins cause growth in the internodes by stimulating cell elongation and cell division. Also promotes seed germination.

Fruit production

  • Gibberellins delay senescence in citrus fruits, extending the time fruits can be left unpicked, making them avalible for longer.
  • Gibberellins acting with cytokinins can make apples elongate to improve their shape.
  • Without gibberellins, bunches of grapes become very compact, this restricts the growth of individual grapes.


  • Adding gibberellins speeds up the process of breaking down starch into maltose.

Sugar production

  • Spraying sugar cane with gibberellins stimulates growth between nodes, making the stem elongate. This can increase sugar yeild as the sugar is stored in the cells of the internodes.

Plant breeding

  • Gibberellins can induce seed formation in young trees.
  • Gibberellins can be added to biennial plants to induce seed production in the first year.
  • Spraying plants with gibberellin synthesis inhibitors can keep flower short and stocky, and ensure that that internodes of crop plants stay short, helping to prevent lodging.
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Cytokinins can delay leaf senescene, they are used to prevent yellowing of lettuce leaves after they have been picked.

They are used in tissue culture to help mass-produce plants. 

They promote bud and shoot growth from small pieces of tissues taken from parent plants. This produces a short shoot with a lot of side branches, which can be split into lots of small plants and grown separately.

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Scientists have developed 2-chloroethylphosphoric acid, which can be sprayed in solution, is easily absorbed and slowly releases ethene inside the plant. Commercial uses of ethen include:

  • Speeding up fruit ripening in apples, tomatoes and citrus fruit.
  • Promoting fruit drop in cotton, cherry and walnut.
  • Promoting female sex expression in cucumbers, reducing the chance of self-pollination and increasing the yeild.
  • Promoting lateral growth in some plants, yeilding compact flowering stems.

Restricting ethene's effects can prevent fruit ripening so it can be stored for longer. It can also increase the shelf life of cut flowers.

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Artificial auxins can be used to prevent leaf and fruit drop, and to promote flowering. In high concentrations auxins can also promote fruit drop.

  • Dipping the end of a cutting in rooting powder before planting encourages root growth.
  • Treating unpollinated flowers with auxin can promote growth of seedless fruit.
  • Artificial auxins are used a herbicides to kill weeds.
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The structure of the human brain - cerebrum, corpu

The cerebrum is the largest part of the human brain, it is responsible for the elements of the nervous system that are associated with being 'human', including thought, imagination and reasoning. It is divided into two hemispheres that are connected via the corpus callosum.The outermost layer which is folded and consists of a thin layer of nerve cell bodies know as the cerebral cortex.

The cerebrum is in control of higher brain functions including:

  • Concious thought.
  • The ability to override some reflexes.
  • Features associated with intelligence, such as reasoning and judgement.

Ther cerebral cortex is subdivided into areas responsible for specifi activites and body regions:

  • Sensory areas recieve impulses indirectly from the receptors.
  • Association areas compare input with previous experience in order to interpret what the input means and judge an appropriate response.
  • Motor areas send impulses to effectors

Motor areas on the left side control the muscular movements of the right side of the body.

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Coordinated motor responses involve the cerebellum

The cerebellum controls the coordination of movementand posture.

Neurones from the cerebellum carry impulses to the motor areas so that motor output to the effectors can be adjusted appropriately in relation to what is required from them.

In order to coordinate balance and fine movement the cerebellum needs to process information from the following locations:

  • The retina.
  • The balance organs in the inner ear.
  • Specialised fibres in muscles called 'spindle' fibres which give information about muscle tension.
  • The joints.
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Medulla oblongata

The medulla oblongata controls the action of smooth muscle in the gut wall and controls breathing movements and heart rate.

Controls non-skeletal muscles. This means that it is effectively in control of the autonomic nervous system. Regulatory centres for a number of vital processes are found in the medulla oblongata including:

  • The cardiac center, which regulates heart rate.
  • The respiratory centre, which controls breathing and regulates the rate and depth of breathing.
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The hypothalamus controls the autonomic nervous system and the endocrine glands.

Controls most of the body's homeostatic mechanims. Sensory input from temperature receptorys and osmoreceptors is recieved by the hypothalamus and leads to the initiation of automatic responses that regulate body temperature and blood water potential. The hypothalamus also controls much of the endocrine function of the body because it regulates the pituitary gland.

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The CNS and PNS

The central nervous system consists of the brain and spinal cord. It is made up of grey matter (billions of non-myelinated nerve cells) and white matter (longer, myelinated axons and dendrons that carry impulses). The presence of myelin makes the long fibres appear white.

The peripheral nervous system is made up of the neurones that carry impulses into and out of the CNS.

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Sensory and motor systems of the peripheral nervou

Sensory neurones carry impulses from the many receptors, in and around the body, to the CNS. Motor neurones carry impulses from the CNS to the effector organs. Many neurones are bundled together and covered in connective tissue to form nerves. The motor system is further subdivided:

  • Somatic motor neurones carry impulses from the CNS to the skeletal muscles, which are under voluntary (conscious) control.
  • Autonomic motor neurones carry impulses from the CNS to cardiac muscle, to smooth muscle in the gut wall and to glands, none of which are under voluntary control.
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The autonomic nervous system

The system operates to a large extent independently of conscious control and is responsible for controlling the majority of homeostatic mechanisms and so plays a vital role in regulating the internal enviroment of the body within set parameters. It is also capable of controlling the heightened responses associated with the stress response. 

The autonomic nervous system differes from the somatic nervous system in a number of ways:

  • Most autonomic neurones are non-myelinated whilst most somatic neurones are myelinated.
  • Autonomic connections to effectors always consist of at least 2 neurones that connect at a ganglion. Somatic connections to effectors consist of only 1.
  • Autonomic motor neurones occur in two types: sympathetic and parasympathetic.
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Sympathetic and parasympathetic subsystems

Sympathetic and parasympathetic sysms differe in bothe structure and action, they are antagonistic systems.


  • Most active in sleep and relaxation.
  • The neurones of a pathway are linked at a ganglion within the target tissue, so pre-ganglionic neurones vary considerably in length.
  • Post-ganglionic neurones secrete acetylcholine as the neurotransmitter at the synapse between neurone and effector.
  • Effects of action include: decreased heart rate, pupil constriction, decreased ventilation rate and sexual arousal.


  • Most active in times of stress.
  • The neurones of a pathway are linked at a ganglion just outside the spinal cord, so pre-ganglionic neurones are very short.
  • Post-ganglionic neurones secrete noradrenaline at the synapse between neurone and effector.
  • Effects of action include: increased heart rate, pupil dilation, increased ventilation rate and ******.
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Action of muscles

Muscles are only capable of producing a force when they contract, so the movement of any bone at a joint requires the coordinated action of at least two muscles. As one muscle is stimulated to contract, the other muscle of the pair must relax to allow from smooth movement. Muscles working in pairs are describes as antagonistic. However, the movemnt of bones at many joints requires a wider range of actions and is under the control of groups of muscles called synergists.

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Movement at the elbow joint

The elbow joint is a synovial joint. These joints occur where a large degree of movement is required. The synovial fluid is a lubricant which eases the movement of the bones at the joint. The biceps and tricepts muscles act antagonistically in order to move the forearm at the elbow.

  • Impulses arriving at the neuromuscular junction cause vesicles to fuse with the pre-synaptic membrane and release acetylcholine into the gap.
  • Acetylcholine binds to receptors on the muscle fibre membrane causing depolarisation.
  • Depolarisation wave tavels down the tubules (T system).
  • T system depolaristion leads to Ca2+ being released from store in sarcoplasmic reticulum.
  • Ca2+ binds to proteins in the muscles which leads to contraction.
  • Acetylcholinesterase in the gap rapidly breaks down acetylcholine so the contraction only occurs when impulses arrive continuously.
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The motor unit

The brian controls the strength of contraction because many motor neurones stimulate a single muscle. Each one branches to neuromuscular junctions, causing the contraction of a cluster of muscle cells (the motor unit). The more motor units stimulated, the greater the force of contraction. This is known as gradation of response.

neuromuscular junction is a specialised synapse which occurs at the end of a motor neurone where it meets the muscle fibre.

A single electrical stimulus of sufficent strength causes a muscle twitch, increasing the strength of the stimulus increases the force of contraction of the twitch up to a maximal response.

Two large, separate stimuli, if far apart give two separate twitches, each to the maximal response. However, if the stimuli are applied close together the response becomes overlapped and is more powerful than a single maximal response. This is called summation.

Repeated large stimuli give a sustained and powerful contraction know as a tetanus.

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Involuntary (smooth) muscle

Smooth muscle is innervated by neurones of the autonomic nervous system.


  • Walls of the intestine - moves food along the intestine
  • Iris of the eye - controls the intensity of light entering the eye
  • Walls of arterioles; wall and cervix of uterus - important in regulation of temperature and blood pressure. Important in redirecting the blood to voluntary muscle during exercise.


  • Not striated.
  • 'spindle-shaped'
  • Contain small budles of actin and myosin, and a single nucleus
  • Cells in the relaxed state are about 500um long and 5um wide.
  • Contraction is relatively slow but this muscle also tires slowly
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Cardiac muscle

Cardiac muscle forms the muscular part of the heart. There are three types:

  • atrial muscle
  • ventricular muscle
  • specialised excitatory and conductive muscle fibres.

Atrial and ventricular muscle contract in a way similar to that of skeletal muscle but with a longer duration of contraction. The excitatory ams conductive fibres contract feebly but conduct electrical impulses and control the rhythmic heartbeat.

Some cardiac muscle fibres are capable of stimulating contraction without a nerve impulse, this type of contraction is myogenic. However, neurones of the autonomic nervous system carry impulses to the heart to regulate the rate of contraction. Sympathetic stimulation increases its rate, whereas parasympathetic stimulation decreases it.

The SAN in the wall of the right atrium is made of specialised excitatory and conductive fibres. It has the greatest ability for self-excitation and the electrival activity generated immediately spreads into the atrial wall. The AV conducts the activity to the ventirical tips.


  • Striated.
  • Made of many individual cells connected in rows.
  • Dark areas are intercalated discs which are cell membranes which have fused in such a way that there are gap junctions with free diffusion of ions and so action potentials pass very easily and quickly between cardiac muscle fibres.
  • Contracts powerfully and without fatigue.
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Voluntary (skeletal) muscle

The action of voluntary muscles leads to movement of the skeleton at the joints. 


  • Muscle cells form fibres containg several nuclei.
  • Each fibre is surrounded by a cell surface membrane called the sarcolemma.
  • Muscle cell cytoplasm is known as sacroplasm.
  • Many mitochondria.
  • An extensive sarcoplasmic reticulum.
  • A number of myofibrils.
  • Contracts quickly and powerfully, but it fatigues quickly.
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The sacromere

The sacromere in voluntary muscle is the span from one z-line to the next.

Z-lines are closer together during contraction because the lengths of the I-band and H-zone are reduced. The A-band doesn't change in length during contraction. 

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Protein filaments involved in contraction

There are two types of protein filaments found in muscle cells:

  • Thin filaments are two strands, made chiefly of the protein actin (F actin), coiled around each other. Each strand is composed of G actin (globular protein) subunits. Tropomyosin (a rod-shaped protein) molecules coil around the F actin, reinforcing it. A troponin complex is attached to each tropomyosin molecule. Each troponin complex consists of three polypeptides, one binds to actin, one binds to tropomyosin (keeping it in place around the actin filament) and one binds to calcium ions.
  • Thick filaments are bundles of myosin. Each myosin molecule consists of a tail and two protruding heads. Each thick filament consists of many myosin molecules whose heads stick out from opposite ends of the filament.
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The power stroke

  • Myosin head groups attach to the surrounding actin filaments forming a cross-bridge.
  • The head group then bends, causing the thin filament to be pulled along and so overlap more with the thick filament. This is the power stroke. ADP and Pi are released.
  • The cross-bridge is then broken as new ATP attaches to the myosin head.
  • The head group moves backwards as the ATP is hydrolysed to ADP and Pi. It can then form a cross-bridge with the thin filament further along and bend again.

In a contracting muscle, several million cross-bridges are continuously being made and broken, causing the thin filaments to slide past the thick filaments and so shorten the sacromere. This shortens the whole length of the muscle.

cross-bridge is the name given to the attachment formed by a myosin head binding to a binding site on an actin filament.

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Calcium ions

The binding sites for the myosin head group on the actin fibre are covered by the tropomyosin subunits. This prevents cross-bridges being formed as the head group can't bind to the binding site.

When an action potential arrives via a neurone at the neuromuscular junction, calcium ions are released from the sarcoplasmic reticulum in the sarcomeres. These ions diffuse through the sarcoplasm and bind to the trophin molecules. This changes the shape of the trophin molecules, which moves the tropomyosin away from the binding sites on the actin. The acting-myosin binding sites are uncovered and so cross-bridges can form. This allows the power stroke and muscle contraction to occur.

When neurostimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum by carrier proteins on the membrane. This leads to muscle relaxation.

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Role of ATP in the power stroke

When the myosin head attaches to the actin binding site and 'bends', the molecules are in their most stable form.

Energy from ATP is required in order to break the cross-bridge and re-set the myosin head forwards.

The myosin head group can then attach to the next binding site along the actin molecule and bend again.

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Coordination of the physiological changes

  • When a threat is percieved the cerebal understanding of a threat stimulates the hypothalamus.
  • The hypothalamus stimulates increased activity in the sympathetic nervous system and triggers the release of adrenaline from the adrenal medulla into the blood. The hypothalamus also releases CRF into the pituitary gland stimulating the release of ACTH from the anterior pituitary gland.

ACTH stimulates the release of a number of different corticosteroid hormones from the adrenal cortex, some of which help the body to resist stressors.

The combined effects of increased sympathetic nervous system activity and the release of adrenaline and other hormones into the blood are responsible for the physiological changes of the 'fight or flight' response.

The fight or flight response is the set of responses in an animal that accompany the perception of threat. The response is driven by the sympathetic nervous system and sets the body at a higher level of capacity to respond to the threat.

stressor is a stimulus that causes the stress response. It causes wear and tear on the body's physical or mental resources.

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Innate behaviour

Innate behaviour is any animal response that occurs without the need for learning. It is an inherited response, similar in all members of the same species and is always preformed in same way in response to the same stimulus.


Reflexes are involuntary responses which follow a specific pattern in response to a given stimulus.


An orientation behaviour where the rate of movement increases when the organism is in unfavourable conditions. The behaviour is 'non-directional' meaning that the response is to change the rate of movement overall in relation to the intensity of the stimulation, not in any particular direction.


A taxis is a 'directional' orientation response. The direction of movement is described in relation to the stimulus which triggers the behavioural response.

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Learned behaviour 1

Learned behaviour refers to animial responses that change or adapt with experience.


Animals learn to ignore certain stimuli because repeated exposure to the stimulus results in neither a reward nor punishment. It is important in screening out many non-dangerous stimuli in the enviroment and avoids wasting energy in making escape responses to non-harmful stimuli.


This involves young animals imprinting on another organism, after that they will only follow and learn from organisms that look like the first organism. It only occurs in a sensitive period. Imprinting is essential to helping young to learn skills from the parents. Imprinting was demonstrated by Konrad Lorenz..

Latent learning

Animals will explore new surroundings and retain information about their surroundings that isn't of immediate use to them but may be essential to staying alive in the future.

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Learned behaviour 2

Classical conditioning

Was described by Pavlov. Before conditioning an unconditioned stimulus gives an unconditioned response and a neutral stimulus will give no conditional response. During conditioning the unconditioned stimulus and the neutral stimulus are present at the same time and produce the unconditioned response. After conditioning the neutral stimulus become a conditioned stimulus and will produce a conditioned response.

Operant conditioning

Was described by B.F.Skinner. An animal is stimulated by a reward, this will lead to increasing frequency of the behaviour that gained the reward. A variety of rewards and reinforcers can be used. Also known as trial and error learning.

Insight learning

Highest form of learning. Based on the ability to think and reason in order to solve problems or deal with situations in ways that do not resemble reflec responses or trial and error. Once solved the solution is remembered. Wolfgang Kohler.

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